Water-based pigment dispersions are well known in the art, and have been used commercially for applying films, such as paints, to various substrates. The pigment dispersion is generally stabilized by either a non-ionic or ionic technique. When using the non-ionic technique, the pigment particles are stabilized by a polymer that has a water-soluble, hydrophilic section that extends into the water and provides entropic or steric stabilization. Representative polymers useful for this purpose include polyvinyl alcohol, cellulosics, ethylene oxide modified phenols, and ethylene oxide/propylene oxide polymers. While the non-ionic technique is not sensitive to pH changes or ionic contamination, it has a major disadvantage for many applications in that the final product is water sensitive. Thus, if used in ink applications or the like, the pigment will tend to smear upon exposure to moisture.
In the ionic technique, the pigment particles are stabilized by a polymer of an ion containing monomer, such as neutralized acrylic, maleic, or vinyl sulfonic acid. The polymer provides stabilization through a charged double layer mechanism whereby ionic repulsion hinders the particles from flocculation. Since the neutralizing component tends to evaporate after application, the polymer then has reduced water solubility and the final product is not water sensitive.
Ideally, a polymer dispersant that provides both steric and charged double layer stabilization forces would make a much more robust pigment dispersion. Polymer dispersants having both random and block structures have been disclosed for this purpose. For example, U.S. Pat. No. 4,597,794 to Canon proposes an aqueous ink dispersion for ink jet printers in which a pigment is dispersed by a polymer having ionic hydrophilic segments and aromatic hydrophobic segments that adhere to the pigment surface. U.S. Pat. No. 5,085,698 to Ma et al. discloses the use of AB or BAB structured block polymers as pigment dispersants. While the random polymer dispersants can be prepared readily by conventional polymerization techniques, the structured block polymer dispersants generally offer improved dispersion stability. However, the structured block polymers usually require a more elaborate synthesis plan, more demanding reaction conditions, and high raw materials purity, as in the case of anionic polymerization and group transfer polymerization. A more convenient polymerization process, whereby desired structure may readily be built into the polymer, is desired for more advanced high quality dispersion applications such as ink jet printers.
Ink jet printing is a non-impact printing process wherein a digital signal produces ink droplets on media substrates, such as paper or transparency films. In thermal ink jet printing, resistive heating is used to vaporize the ink, which is expelled through an orifice in the ink jet printhead toward the substrate. This process is known as firing, in which water is vaporized by the heat, causing a very rapid and drastic local compositional change and temperature rise. This occurs repeatedly at high frequency during the life of the printhead. Further, in the orifice areas, the ink composition can drastically change from water-rich to solvent-rich due to evaporation of water. This tends to cause the pigment particles to flocculate around the orifice in the printhead, eventually leading to plugging of the orifice. Particle flocculation thus will cause misdirection of the ink drop, or prevent the drop to eject at all in extreme cases.
In conventional coating applications, many additives of organic nature are used to impart the desired physical properties for the final use. Examples include polymer binders, thickeners, thixotropic agents, coating aids, etc. During the drying process, these components are concentrated. The pigment dispersion needs to accommodate such changes in order to maintain the uniformity and color quality for the final coatings.
Accordingly, the need exists for dispersants which can be prepared by a convenient and inexpensive polymerization process, and will provide improved dispersion stability for improved quality for applications such as thermal ink jet printers.